1. Field of the invention
This invention relates to a matrix display unit which is suitable to a large size and high resolution display, and more concretely to a matrix display using nonlinear diodes.
2. Description of the Prior Art
Majority of the conventional matrix display units using nonlinear diodes as drive elements use liquid crystal as the display medium.
Applications of the liquid crystal display (LCD) units have been expanded from watches, calculators, etc. to computer terminals and picture displays. Thus, they have been required to be higher in resolution and larger in display area. There have been known passive-matrix addressing type and active-matrix addressing type LCDs. The latter type includes a type which uses transistors such as thin film transistors (TFTs) and a type which uses nonlinear diodes. Each method has its own advantages and disadvantages. The passive-addressing type is poor in resolution, particularly when a large number of scanning electrodes are used. The transistor driving type requires a complicated manufacturing process, thus being higher in cost. The type using nonlinear diodes is easier to manufacture than the type using transistors, which contributes to cost reduction, and is suitable to construct a large panel.
The matrix liquid crystal display unit using nonlinear diodes mainly includes the following two kinds:
A first one is of a type which uses metal-insulator-metal(MIM) devices. ["The Optimization of Metal-Insulator-Metal devices for use in Multiplexed Liquid Crystal Displays", D. R. Baraff et al, IEEE Transaction of Electron Devices, vol. ED-28, No. 6 pp 736-739, June, 1981]. Tantalum oxide (Ta.sub.2 O.sub.5)has been most frequently used as an insulator. The nonlinearity of the voltage-current characteristic is attributed to the Poole-Frankel effect. A second one is of a type which uses diode rings ["A LCTV Display Controlled by a Si Diode Rings", S. Togashi, et al, SID 84 Digest, 1984, 18. 6, pp. 324-325]. The diode ring is composed of two amorphous silicon (a Si) PIN diodes which are connected in a ring form. This nonlinear diode utilizes the forward characteristic of the PIN diode.
The use of these nonlinear diodes makes it possible to reduce a crosstalk voltage, so that a large-scale display capacity can be realized when compared with the passive-matrix addressing type.
Since each nonlinear diode must be applied with an adequately high voltage to drive each pixel element, it must be designed to have an electric capacitance not exceeding about one-tenth of that of the pixel element. The MIM device is made smalled-sized because of the large relative dielectric constant of tantalum oxide (at least 20), which causes the yield decrease greatly. In addition, it requires during manufacturing at least three photolithography steps, which are complex to operate as well as require a lot of time to finish.
The diode ring requires at least five to six photolithography steps during manufacturing, which results in reduced yield and increased cost.